Method and system for split-pair reception in twisted-pair communication systems

ABSTRACT

A method and system are disclosed for improving the performance of a multiline transmission system by using one or more split-pair receivers in a multiline communications system to identify crosstalk on a pair of transceivers coupled to the split pair receivers, wherein each split pair receiver receives a signal including the crosstalk from each transceiver and provides a corresponding signal vector to a post processing unit, and performing MIMO post-processing on signal vectors received at a receiver from each transceiver and each split-pair receiver while minimizing crosstalk on pairs of lines in the multiline communications system with a frequency equalizer.

This application claims the benefit of the filing date of the followingProvisional U.S. Patent Application:

Split-Pair Signals in Communications Systems, U.S. ProvisionalApplication Ser. No. 60/412,160, filed on Sep. 19, 2002.

FIELD OF THE INVENTION

The present invention relates generally to copper-based communicationsystems and, in particular to, multiple line transmission incopper-based communication systems.

BACKGROUND OF THE INVENTION

The explosive popularity of the Internet has ushered in a new era fortelecommunications. The expanding use of applications such as e-mail,online services, secure electronic transactions, media file sharing,video conferencing, remote collaboration, and telecommuting, to name buta few, is causing a significant increase in the bandwidth needs anddemands of telecommunications users.

As a result, service providers around the world are continuouslysearching for economically attractive ways of deploying newhigh-bandwidth, high-revenue services. Over the last few years,telephone, cable, and wireless operators have been upgrading theirequipment in order to utilize their existing infrastructures forhigher-bitrate services. The result has been the widespread availabilityof high-speed data connectivity through Digital Subscriber Line (DSL)service, high-speed digital cable, and, more recently, Third Generation(3G) wireless networks.

Copper telephone lines are generally viewed as the most widely availableaccess medium that is suitable for high-speed data connectivity. In animpressive display of technological progress, the lines that until a fewyears ago carried only low-speed voice services, are now used to deliverservices running at speeds up to several Mbps (Mega-bits per second).This progress has caused the existing copper infrastructure to be viewedas a significant capital asset that telephone carriers can utilize tomeet the increasing bandwidth demands of their customers.

Over the last two decades, several technologies for data transmissionover copper lines have been developed, including, but not limited tothese technologies: T1/E1, ISDN (integrated services digital network),HDSL (High Speed DSL), SDSL (Symmetric DSL), ADSL (Asymmetric DSL), VDSL(Very High Speed DSL) and ADSL2. These technologies have continued toincrease the data throughput that can be delivered over copper, but thatprogress is now being slowed by the shortcomings of the existing copperinfrastructure.

Typically, a copper line consists of two copper wires that are twistedtogether to produce a “twisted copper pair.” Multiple twisted pairs arethen twisted together into bundles called “binders.” Twisting of pairswas not used in the early days of voice telephony. It was introducedlater in an effort to reduce the effect of interference noise on thereceived signal. The logic behind this noise reduction is as follows:Interference noise on each copper pair is caused by sources outside thecopper pair, such as services operating on other copper pairs in thesame binder or in adjacent binders, radio towers, power wires,electrical appliances at the customer's premises, etc. Those sourcestransmit signals whose electromagnetic fields generate noise voltagesignals on each copper wire. If the two wires were not twisted together,the electromagnetic fields from the noise sources could potentially havevery different values on each of the two wires, and thus they couldgenerate very different noise voltage signals on each of the two wires.By twisting the two wires together in a copper pair, the electromagneticfields from the noise sources have roughly the same value on both wires,and therefore they generate noise voltage signals that are approximatelyequal on both wires. Therefore, when these approximately equal noisesignals are subtracted at the differential receiver of the copper pair,the resulting differential noise voltage signal is much smaller than itwould have been had the two wires not been twisted. On the other hand,the main communications signal, which is transmitted as the voltagedifference between the two wires of the copper pair, is not affected bythe twisting of the wires and is received by the differential receiverin full strength. Therefore, the use of twisted copper pairs anddifferential receivers reduces the effect of the noise only, and doesnot have a similar reduction effect on the main signal. As a result, theSNR (Signal-to-Noise Ratio), and thus also the communications capacity,that can be achieved on a twisted copper pair with a differentialtransmitter and receiver is substantially higher than the SNR of anuntwisted copper pair of the same gauge and length with a common-mode(i.e., non-differential) transmitter and receiver.

When telephone carriers began setting the specifications for the copperpairs used in their cables, including the gauge and twisting of thewires, they considered only POTS (Plain-Old Telephone Service) voiceservices, which typically occupy only a small frequency band at the lowend of the spectrum, approximately from 0 to 4 kHz. As a result, theydecided to use relatively thin wires, mostly 26AWG and 24AWG, and totwist them once every few feet or so. At the time, these choices ofgauge and twist length represented an excellent tradeoff betweenperformance and cost. Wires of larger gauges that are twisted moretightly cost more to produce and are heavier, larger, and less flexible;therefore, they also cost more to transport and to install. Moreover,having enough copper pairs to serve all the current and future customersin a given area means that, as the wires become larger and heavier, theunderground conduits that carry them have to be larger, and the polesthat support them have to be sturdier. Since POTS service utilizesfrequencies below 4 kHz, these choices of gauge and twist length wereperfectly adequate for ensuring high-quality POTS service with very lowcrosstalk noise. Crosstalk originally denoted the noise that wasgenerated when the voice of one telephone subscriber talking on his line“crossed” into another subscriber's line and could be heard aslow-volume background speech.

Unfortunately, these choices do not guarantee the same performance whenthe copper pairs are used to deliver high-speed data services, whichtypically operate at frequencies that are more than 100 times higherthan POTS frequencies. The small gauge of the wires results in highattenuation of high-frequency signals; as a result, the data capacity ofa copper pair decreases rapidly as the length of the pair increases.Even worse, the long twisting of existing copper pairs is much lesseffective at reducing crosstalk noise at higher frequencies. As thefrequencies used for transmission and reception of signals rise, thewavelengths of those signals are reduced. Therefore, pairs with longtwisting appear more and more as untwisted pairs to the correspondingshort-wavelength electromagnetic fields. Since the cost of replacingexisting copper pairs with new ones that have much shorter twistingwould be prohibitively high, the only economically viable option is touse existing pairs for the transmission of high-speed data services. Asa result, high-speed data services are much more sensitive to crosstalknoise from outside sources. This is especially detrimental on longerlines, since the main signal on those lines is already significantlyattenuated; as a result, the increased crosstalk noise often reduces theSNR to levels that are not suitable for high-speed data transmission.

The realization that crosstalk is one of the primary causes ofperformance degradation in high-speed data transmission over copperpairs has resulted in substantial attention to the problem of mitigatingcrosstalk. Transmission technologies designed for operation over asingle copper pair attempt to mitigate crosstalk through the useadvanced coding schemes and adaptive filters that aim to maximize SNR ina given crosstalk environment. Such mitigation techniques have evolvedsteadily over the last decade, and have reached a level of maturitywhere small additional SNR gains come at the cost of significantincreases in complexity.

The most recent advance in the effort to mitigate crosstalk on copperpairs is the use of “vectoring” in multiline transmission schemes. Insuch schemes, multiple copper pairs are used to deliver high-speedservices; but instead of simply using each copper pair as a separatecommunications channel and “bonding” the corresponding data streams atthe digital layer, vectoring techniques coordinate the transmissionand/or reception of signals at the physical layer, in order to increasethe overall capacity of the multiline communications channel. One suchvectoring scheme, disclosed in a recent application PCT/US 03/18004,which is incorporated herein by reference, exploits the correlation ofcrosstalk noise across its associated multiple copper pairs. Inparticular, that scheme treats the transmitters and receivers onmultiple copper pairs as inputs and outputs of a MIMO (Multiple InputMultiple Output) communications channel. Operating in the signal spacedefined by these multiple inputs and outputs, the scheme identifies thesubspace that contains the crosstalk noise, and then uses MIMOpre-processing at the transmitter and MIMO post-processing at thereceiver to transmit most of its main signal in the subspace that isorthogonal to the crosstalk noise. For DMT (Discrete-Multi-Tone)systems, these operations are simply matrix multiplications in thefrequency domain. The MIMO post-processing at the receiver consists ofmultiplying the received symbol vector in each bin by a matrix thatcombines the operations of noise pre-whitening and frequencyequalization. The MIMO pre-processing at the transmitter consists ofmultiplying the transmitted symbol vector in each bin with a matrix thatcompensates for the distortion caused by the noise pre-whitening matrixat the receiver. As a result, the main signal is received withoutdistortion, while the crosstalk noise, which is not multiplied by theMIMO pre-processing matrix, is restricted to a small subspace of thereceived signal space.

Another way of interpreting the vectoring effect of that MIMO scheme isthat it identifies the crosstalk noise on some of its receivers and thenremoves it from the remaining receivers, thereby significantly reducingits overall effect. As a rule of thumb, the effectiveness of this typeof crosstalk mitigation is reduced as the number of strong independentcrosstalk sources increases beyond the number of receivers available inthe multiline system. This is due to the fact that each independentcrosstalk source increases the dimension of the crosstalk subspace byone. As the number of such crosstalk sources increases beyond the numberof receivers available, the dimension of the crosstalk subspace becomesequal to the dimension of the signal space of the multiline system,thereby eliminating the orthogonal subspace where the main signal can bereceived free of crosstalk noise.

Therefore, it would be desirable to find a way to increase the dimensionof the signal space in a multiline system for a given number of copperpairs used.

SUMMARY OF THE INVENTION

A method and system are disclosed for improving the performance of amultiline transmission system by adding “split-pair” receivers tomeasure and process additional signals across copper pairs. Theseadditional “split-pair” receivers increase the dimension of the signalspace and therefore enhance the crosstalk mitigation abilities of themultiline system.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present invention will be apparent to oneskilled in the art in light of the following detailed description inwhich:

FIG. 1 illustrates an exemplary communication system that may benefitfrom the present method and system;

FIG. 2 illustrates an exemplary embodiment of a communication system,wherein the present method and system may be implemented;

FIG. 3 illustrates an exemplary two-pair multiline transmission systemfor reducing crosstalk noise utilizing one AFE (Analog Front End)circuit for each copper pair, for a total of two AFE circuits;

FIG. 4A illustrates various two-pair multiline transmission systemscorresponding to several exemplary embodiments of the present inventionas applied to two-pair multiline transmission systems;

FIG. 4B illustrates a split-pair receiver circuit according to oneembodiment of the present invention;

FIG. 5 illustrates an exemplary functional block diagram of split-pairMIMO processing according to one embodiment of the present invention;

FIG. 6 illustrates an exemplary split-pair MIMO transmission process 600once a processing architecture is computed according to one embodimentof the present invention; and

FIG. 7 illustrates an exemplary block diagram of a computer system 2000representing an integrated multi-processor, in which elements of thepresent invention may be implemented.

DETAILED DESCRIPTION

A method and system are described for improving the performance of amultiline transmission system by receiving additional signals acrosscopper pairs. For purposes of discussing and illustrating the invention,an example of a multiline system using two copper pairs will bedescribed. However, one skilled in the art will recognize and appreciatethat the same methodology can be applied to multiline systems with morethan two copper pairs.

In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. It will be evident, however, toone skilled in the art that the present invention may be practicedwithout these specific details. In some instances, well-known structuresand devices are shown in block diagram form, rather than in detail, inorder to avoid obscuring the present invention. These embodiments aredescribed in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that logical, mechanical, electrical,and other changes may be made without departing from the scope of thepresent invention.

Some portions of the detailed descriptions that follow are presented interms of algorithms and symbolic representations of operations on databits within a computer memory. These algorithmic descriptions andrepresentations are the means used by those skilled in the dataprocessing arts to most effectively convey the substance of their workto others skilled in the art. An algorithm is here, and generally,conceived to be a self-consistent sequence of acts leading to a desiredresult. The acts are those requiring physical manipulations of physicalquantities. Usually, though not necessarily, these quantities take theform of electrical or magnetic signals capable of being stored,transferred, combined, compared, and otherwise manipulated. It hasproven convenient at times, principally for reasons of common usage, torefer to these signals as bits, values, elements, symbols, characters,terms, numbers, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the following discussion,it is appreciated that throughout the description, discussions utilizingterms such as “processing” or “computing” or “calculating” or“determining” or “displaying” or the like, refer to the action andprocesses of a computer system, or similar electronic computing device,that manipulates and transforms data represented as physical(electronic) quantities within the computer system's registers andmemories into other data similarly represented as physical quantitieswithin the computer system memories or registers or other suchinformation storage, transmission or display devices.

In varying embodiments, the present invention can be implemented by anapparatus for performing the disclosed method. The apparatus may bespecially constructed or may comprise a general-purpose computer suchthat when configured by a computer program executes the disclosedmethod. Such a computer program may be stored in a computer readablestorage medium, such as, but not limited to, floppy disks, opticaldisks, CD-ROMs, and magnetic-optical disks, read-only memories (ROMs),random access memories (RAMs), magnetic or optical cards, or any type ofmedia suitable for storing electronic instructions.

The methods of the invention may be implemented using computer software.If written in a programming language conforming to a recognizedstandard, sequences of instructions designed to implement the methodscan be compiled for execution on a variety of hardware platforms and forinterface to a variety of operating systems. In addition, the presentinvention is not described with reference to any particular programminglanguage. It will be appreciated that a variety of programming languagesmay be used to implement the teachings of the invention as describedherein. Furthermore, it is common in the art to speak of software, inone form or another (e.g., program, procedure, application . . . ), astaking an action or causing a result. Such expressions are merely ashorthand way of saying that execution of the software by a computercauses the processor of the computer to perform an action or produce aresult.

The algorithms and displays presented herein are not inherently relatedto any particular computer or other apparatus. Various general-purposesystems may be used with programs in accordance with the teachingsherein, or it may prove convenient to construct a more specializedapparatus to perform the required method. For example, any of themethods according to the present invention can be implemented inhard-wired circuitry, by programming a general-purpose processor or byany combination of hardware and software. One of skill in the art willappreciate that the invention can be practiced with computer systemconfigurations other than those described below, including hand-helddevices, multiprocessor systems, microprocessor-based or programmableconsumer electronics, DSP devices, network PCs, minicomputers, mainframecomputers, and the like. The invention can also be practiced indistributed computing environments where tasks are performed by remoteprocessing devices that are linked through a communications network. Therequired structure for a variety of these systems will appear from thedescription below.

It is to be understood that various terms and techniques are used bythose knowledgeable in the art to describe communications, protocols,applications, implementations, mechanisms, etc. One such technique isthe description of an implementation of a technique in terms of analgorithm or mathematical expression. That is, while the technique maybe, for example, implemented as executing code on a computer, theexpression of that technique may be more aptly and succinctly conveyedand communicated as a formula, algorithm, or mathematical expression.Thus, one skilled in the art would recognize a block denoting A+B=C asan additive function whose implementation in hardware and/or softwarewould take two inputs (A and B) and produce a summation output (C).Thus, the use of formula, algorithm, or mathematical expression asdescriptions is to be understood as having a physical embodiment in atleast hardware and/or software (such as a computer system in which thetechniques of the present invention may be practiced as well asimplemented as an embodiment).

Overview of a General Communication Network

FIG. 1 illustrates an exemplary communication system 105 that maybenefit from the present method and system. The backbone network 120 isgenerally accessed by a user through a multitude of access multiplexers130 such as: base stations, DSLAMs (DSL Access Mulitplexers), orswitchboards. The access multiplexers 130 communicate with the networkusers. The user equipment 140 exchanges user information, such as userdata and management data, with the access multiplexer 130 in adownstream and upstream fashion. The upstream data transmission isinitiated at the user equipment 140 such that the user data istransmitted from the user equipment 140 to the access multiplexer 130.Conversely, the downstream data is transmitted from the accessmultiplexer 130 to the user equipment 140. User equipment 140 mayconsist of various types of receivers that contain modems such as: cablemodems, DSL modems, and wireless modems. In this network access systemthe current method and system may be practiced to identify sources ofinterference in the access channels.

Overview of Exemplary DSL System

“DSL” is to be understood to refer to a variety of Digital SubscriberLine (DSL) standards that, even now, are evolving. Each DSL standardwill be referred to as a DSL service type. At the present time, DSLservice types include, but are not limited to, ADSL, SDSL, HDSL, andVDSL (Asymmetrical, Symmetrical, High speed, and Very high speed DSL,respectively).

FIG. 2 illustrates an exemplary embodiment of a communication systemwherein the present invention may be implemented. A central office 202has a series of DSL modems 204-1 through 204-N, including transceivers214-1 through 214-N, connected via twisted pairs 206-1 through 206-N asa bundle 208 connected to customers DSL 210-1 through 210-N, includingtransceivers 216-1 through 216-N, which is connected respectively tocustomers premise equipment (CPE) 212, such as a local area network orcomputer. One skilled in the art recognizes that twisted pair bundle 208may experience crosstalk between the twisted pairs 206-1 through 206-Nand depending upon the services carried by the pairs, data rates, andother factors, such as proximity of the pairs to each other, etc., theremay be varying types of crosstalk on each pair, such as alien or out ofdomain crosstalk.

Split-Pair Receiver AFE

The present invention discloses a method for improving the crosstalkmitigation capabilities of a multiline system by increasing thedimension of the signal space without increasing the number of copperpairs. By inserting additional receiver circuitry between the wires ofadjacent copper pairs and modifying the multiline transmission scheme toincorporate the additional receiver measurements, it is possible toincrease the dimension of signal space within the multiline system whilemaintaining the existing quantity of copper pairs. This effectively“splits” the copper pairs of the multiline system, and is thereforecalled a “split-pair” receiver.

FIG. 3 illustrates an exemplary two-pair multiline transmission systemfor reducing crosstalk noise utilizing one AFE (Analog Front End)circuit for each copper pair, for a total of two AFE circuits. Themultiline system 301 includes two separate but essentially identical AFEcircuits 311 and 341, each devoted to one of the two copper pairsconnected to the multiline system 301 at Tip1 313, Ring1 315, Tip2 343,and Ring2 345, respectively. Differential receiver lines 302 and 306 areconnected to the multiline 2×2 transceiver 303 for receiving signalsfrom their associated twisted copper pairs 313, 315, and 343, 345,respectively. Differential transmission lines 304 and 308 are connectedto the multiline 2×2 transceiver 303 for transmitting signals through totheir associated twisted copper pairs 313, 315, and 343, 345,respectively.

The multiline 2×2 transceiver 303 includes a digital signal processorwherein MIMO processing can be performed in either the time domain orthe frequency domain as further discussed below in reference to FIGS. 5,6, and 7. The 2×2 designation refers to the number of differentialtransmitter lines, in this case two, 304 and 308, and the number ofdifferential receiver lines, again two, 302 and 306. Further, 2×2defines the size of the matrix associated with the MIMO processingscheme.

Transceiver AFEs 310 and 340 illustrate in more detail the transceivers311 and 341 of multiline transmission system 301. Drive1P 316, Drive1N318, Drive2P 346 and Drive2N 348 further illustrate differentialtransmission lines 304 and 308. Recvr1P 320, Recvr1N 322, Recvr2P 350and Recvr2N 352 further illustrate differential receiver lines 302 and306. Tip1 312 and Ring1 314 illustrate twisted pair 313 and 315,respectively, of multiline transmission system 301 in AFE 310. Tip2 342,and Ring2 344 illustrate twisted pair 343 and 345, respectively, ofmultiline transmission system 301 in AFE 340.

The addition of split-pair receivers enhances the crosstalk mitigationcapability of the multiline system over the two-pair multilinetransmission system. The addition of split-pair circuitry increases thenumber of receivers that can be used to identify independent sources ofcrosstalk noise, and therefore increase the number of receivers fromwhich the crosstalk noise can be effectively removed. This increases thedimension of the crosstalk-free subspace of the multiline system, andtherefore increases the total bitrate that can be supported by themultiline system in a given crosstalk noise environment.

FIG. 4A illustrates various two-pair multiline transmission systemscorresponding to several exemplary embodiments of the present inventionas applied to two-pair multiline transmission systems. Multilinetransmission system 475 has transceiver AFEs 411 and 441 as similarlydescribed in reference to FIG. 3 but with an additional split pairreceiver 471. AFE 411 connects to twisted pair 401 through inputs Tip1413 and Ring1 415. AFE 441 connects to twisted pair 402 through inputsTip2 443 and Ring2 445. The split pair receiver 471 has two inputs thatconnect to Ring1 415 and Tip2 443. Additionally, multiline 2×3transceiver 477 transmits signals to AFE 411 and 441 throughdifferential lines 409 and 439, respectively. Multiline 2×3 transceiver477 also receives signals from AFE 411, 441, and split pair receiver 471through differential lines 407, 437, and 467, respectively. In thiscase, the 2×3 refers to the two differential transmitter lines, 409 and439, and the three differential receiver lines, 407, 437, and 467.Further, 2×3 defines the size of the matrix associated with the MIMOprocessing discussed below with reference to FIGS. 5 through 7.

In another embodiment of the present invention, multiline transmissionsystem 481 has transceiver AFEs 417 and 447, and split pair receiver473, similarly described above with reference to the multilinetransmission system 475. In this embodiment, however, the two inputs ofsplit pair receiver 471 are connected to Ring1 419 and Ring2 449.

In yet another embodiment of the present invention, multilinetransmission system 485 has transceiver AFEs 423 and 453, and split pairreceivers 475 and 477. The two inputs of split pair receiver 475 areconnected to Tip1 425 and Ring2 457 and the two inputs of split pairreceiver 477 are connected to Ring1 427 and Tip2 455. The multiline 2×4transceiver 487 receives differential input 459 from transceiver AFEs423 and 453, and split pair receivers 475 and 477. The additional splitpair receiver 477 adds an additional dimension (a fourth receiver input)to the matrix associated with the MIMO processing discussed below withreference to FIGS. 5 through 7.

FIG. 4B illustrates a split-pair receiver circuit 400, according to oneembodiment of the present invention. There are two separate butessentially identical AFE circuits 410 and 440, each devoted to one ofthe two copper pairs connected to the multiline system at Tip1 412,Ring1 414, Tip2 442, and Ring2 444, respectively. The differential linesDrive1P 416, Drive1N 418, Drive2P 446 and Drive2N 448 are connected tomultiline transceiver circuitry (not shown) for transmitting signalsthrough to their associated twisted copper pairs. The differential linesRecvr1P 420, Recvr1N 422, Recvr2P 450, Recvr2N 452 are connected tomultiline transceiver circuitry for receiving signals through theirassociated twisted copper pairs. Additionally, a split pair receiver 470is connected between AFEs 410 and 440. The split pair receiver 470 is apassive circuit coupled to AFE 410 by a connection at Ring1 414 andanother connection at Tip2 442. Differential lines, Recvr12P 476 andRecvr12N 478, are also connected to the multiline transceiver circuitry.

One of ordinary skill in the art will recognize the additional circuit470 as a passive receiver-only circuit. There is no driver for thesplit-pair receiver circuit 470 because it does not transmit anysignals. The standard transceivers of AFEs 410 and 440 continue totransmit and receive signals in accordance with standard operations. Thecircuit 470 is balanced with respect to circuits 410 and 440, so that itneither affects nor is affected by their transmitted signals.

Further, it should be recognized by one of ordinary skill in the art,that the same concept is applicable to multiline systems that use morethan two copper pairs. In particular, in a multiline system that uses Ncopper pairs, one can add up to N-1 split-pair receivers and process atotal of 2N-1 received signals. These split-pair receivers can be addedin many different configurations; for example, in a multiline systemthat uses three copper pairs, up to two split-pair receivers can beadded between Ring1-Tip2 and Ring2-Tip3, or Tip1-Ring2 and Tip2-Ring3,or Ring1-Tip3 and Ring3-Tip2, or Tip1-Ring3 and Tip3-Ring2, orRing1-Ring2 and Tip2-Tip3, and so on.

The reasoning behind limiting to N-1 the number of split-pair receiversin a multiline system that uses N copper pairs is that the total numberof wires is 2N, and one of the wires is considered a reference groundfor the differential receiver scheme, so that the total number ofindependent measurements that can be made on 2N wires is 2N-1. Sincethere are already N standard-pair receivers in the multiline system,adding more than N-1 split-pair receivers would produce measurementsthat are not independent. Nevertheless, one can use more than N-1split-pair receivers to obtain repeated measurements of the main signaland the crosstalk noise. For example, the embodiment 485 illustrated inFIG. 4A shows a two-pair multiline system with two split-pair receivers475 connected to Tip1 425 and Ring2 457, and 477 connected to Ring1 427and Tip2 455. In general, in a multiline system that uses two copperpairs, one could add up to four split-pair receivers: Ring1-Tip2,Ring2-Tip1, Ring1-Ring2, and Tip1-Tip2. This addition would reduce theeffect of the thermal background noise, which is independent on eachreceiver, and would therefore result in improved measurements of themain signal and the crosstalk noise. However, since additional receiverscome at an additional component cost, it would be up to the systemdesigner to decide whether the small increases in performance that canbe obtained by adding more than N-1 split-pair receivers are enough tojustify the extra cost.

The next step in the effort to enhance the performance of multilinesystems would be to consider each wire in a copper pair as independent,and use it to transmit a separate signal. This scheme would result inthe transmission of up to 2N-1 independent main signals over a multilinesystem that uses N copper pairs, and would further increase the datacapacity of such a multiline system. However, the implementation of sucha split-pair transmission scheme is not practical in the publictelephone network, since there are strict regulations that severelylimit the transmission of common-mode signals over copper pairs, inorder to prevent excessive interference to the installed base ofservices using differential receivers. On the other hand, the additionof split-pair receivers, as disclosed in the present invention, is apassive receiver-only enhancement, and as such it is not limited bypublic telephone network regulations.

Split-Pair MIMO Processing

As is the case with MIMO processing schemes that use only standardreceivers across copper pairs, the claimed method and system forsplit-pair receivers in the present invention is applicable to bothfrequency-domain transmission schemes, such as the Discrete Multi-Tone(DMT) scheme defined in the G.dmt and G.lite standards of theInternational Telecommunications Union (ITU), and time-domaintransmission schemes, such as the 2B1Q scheme defined in the HDSLstandard of the American National Standards Institute (ANSI). Thepreferred embodiment is with frequency-domain schemes, because thesplit-pair MIMO processing can be performed much more efficiently in thefrequency domain, where it reduces to a simple matrix multiplication ineach of the independent frequency bins. Frequency bins are the smallfrequency bands surrounding each individual “tone” or frequency carrierin a DMT system.

FIG. 5 illustrates an exemplary functional block diagram 500 ofsplit-pair MIMO (Multiple Input Multiple Output) processing according toone embodiment of the present invention. Referring to the block diagramof FIG. 5, we use the following notations:

k denotes the kth DMT symbol;

n_(s) denotes the number of elements in the transmitted signal vector,which is equal to the number of transmitters used in the multilinesystem;

n_(r) denotes the number of elements in the received signal vector,which is equal to the total number of receivers used in the multilinesystem, including standard receivers and split-pair receivers;

n_(d) denotes the number of independent crosstalk noise sources thataffect the multiline system;

w(k) 505 is a vector of symbols for a single frequency bin, which hasn_(s) elements;

x(k) 525 is a vector of signals to be transmitted in a single frequencybin after MIMO transmitter pre-processing, which has n_(s) elements;

y(k) 535 is a vector of received signals in a single frequency binbefore MIMO receiver post-processing, which has n_(r) elements;

z(k) 545 is a vector of symbols to be decoded in a single frequency binafter MIMO receiver post-processing, which has n_(s) elements;

d(k) 515 is the vector of interference signals, which has n_(d)elements;

H_(m) 520 is the main channel matrix for a single frequency bin, whichis of dimension n_(r)×n_(s);

H_(d) 530 is the interference matrix for a single frequency bin, whichis of dimension n_(r)×n_(d);

B 510 is the transmitter MIMO pre-processing matrix for a singlefrequency bin, which is of dimension n_(s)×n_(s); and

A 550 is the receiver MIMO post-processing matrix for a single frequencybin, which is of dimension n_(s)×n_(r).

The present invention assumes that the prefix is long enough and thetime equalizer (TEQ) is designed well enough that there is noInter-Block Interference (IBI), that H_(m) is a full-rank matrix (thisis always true if the multiline system uses exactly one transmitter percopper pair and at least one receiver per copper pair), and that theelements of d(k) 515 are random variables with E└d(i)d^(H)(j)┘=R_(d) fori=j and 0 otherwise, implying that the interference noise isuncorrelated between different symbols; one skilled in the art willrecognize that this is only approximately true in practice, but thatthis approximation is a very good one for all practical purposes.

The pair of matrices A 550 and B 510 are computed with the followingproperties:

1. B 510 is Hermitian, so that the transmitted signal power is preservedacross pairs by the pre-processing operation;

2. AH_(m)B=I, so that the symbol vector component of the received signalis the same as the transmitted symbol vector, this allows the receivedsignal to be sliced correctly after receiver post-processing; and

3. the sum of the SNRs (expressed in dB) of the independent receivedsignals, i.e., the elements of the received symbol vector z(k), ismaximized.

The algorithm that solves this problem is as follows:

Step 1: C is computed by solving the equationCH _(d) R _(d) H _(d) ^(H) C ^(H) =I.This implies that that E└CH_(d)dd^(H)H_(d) ^(H)C^(H)┘=I. The resultingmatrix C is a square matrix of dimension n_(r)×n_(r).

Step 2: The n_(r)×n_(s) matrix U and the n_(s)×n_(s) matrices Σ and Vare computed as the solutions of the SVD (Singular Value Decomposition)equationUΣV ^(H) =CH _(m).

Step 3: Then B=V and A=Σ⁻¹U^(H)C.

This solution has the following properties:

Property 1 is satisfied by Step 3, since the matrix V is unitary.

Property 2 can be verified by substituting the equalities in Step 2 andStep 3 into the expression of the property:AH _(m) B=(Σ ⁻¹ U ^(H) C)H _(m) V=V ^(H)(VΣ ⁻¹ U ^(H))(CH _(m))V=V^(H)(CH _(m))^(H)(CH _(m))V=I.

As for Property 3, it can be verified as follows. Using the expressionin Step 1, the noise covariance after MIMO post-processing at thereceiver of the multiline system is${E\left\lfloor {{AH}_{d}{\underset{\_}{dd}}^{H}H_{d}^{H}A^{H}} \right\rfloor} = {{E\left\lfloor {\sum\limits^{- 1}{U^{H}{CH}_{d}{\underset{\_}{dd}}^{H}H_{d}^{H}C^{H}U\quad\sum\limits^{- 1}}} \right\rfloor} = {{\sum\limits^{- 1}{U^{H}E\left\lfloor {{CH}_{d}{\underset{\_}{dd}}^{H}H_{d}^{H}C^{H}} \right\rfloor U\sum\limits^{- 1}}} = {{\sum\limits^{- 1}{U^{H}{CH}_{d}R_{d}H_{d}^{H}C^{H}U\sum\limits^{- 1}}} = {\sum\limits^{- 2}.}}}}$

Consider now a tall matrix W of the same dimensions as U such thatU^(H)W=0. We can define a modified MIMO post-processing matrix Ã asfollows: Ã=Σ⁻¹ (U+W)^(H)C. This implies that ÃH_(m)B=I. The noisecovariance after MIMO post-processing at the receiver of the multilinesystem with the modified MIMO post-processing matrix Ã is${E\left\lfloor {\overset{\sim}{A}H_{d}{\underset{\_}{dd}}^{H}H_{d}^{H}{\overset{\sim}{A}}^{H}} \right\rfloor} = {{E\left\lfloor {\sum\limits^{- 1}{\left( {U + W} \right)^{H}{CH}_{d}{\underset{\_}{dd}}^{H}H_{d}^{H}{C^{H}\left( {U + W} \right)}\sum\limits^{- 1}}} \right\rfloor} = {{\sum\limits^{- 1}{\left( {U + W} \right)^{H}E\left\lfloor {{CH}_{d}{\underset{\_}{dd}}^{H}H_{d}^{H}C^{H}} \right\rfloor\left( {U + W} \right)\sum\limits^{- 1}}} = {{\sum\limits^{- 1}{\left( {U + W} \right)^{H}{CH}_{d}R_{d}H_{d}^{H}{C^{H}\left( {U + W} \right)}\sum\limits^{- 1}}} = {{\sum\limits^{- 1}{\left( {U + W} \right)^{H}\left( {U + W} \right)\sum\limits^{- 1}}} = {\sum\limits^{- 2}{+ {\sum\limits^{- 1}{W^{H}W\sum\limits^{- 1}}}}}}}}}$

Clearly, the covariance of the noise in this system is minimized withW=0, which means that the MIMO post-processing matrix A satisfiesProperty 1.

The split-pair MIMO processing architecture of FIG. 5 uses thepre-processing matrix B 510 at the transmitter and the post-processingmatrix A 550 at the receiver to coordinate the transmitted and receivedsignals in a fashion that preserves the integrity of the transmittedsignal w(k) 550, while simultaneously pre-whitening the crosstalk noiseto create a set of directions in the symbol vector space, namely asubspace, that is free of crosstalk noise.

In particular, the interference noise after receiver post-processing isgiven by AH_(d)d(k). As shown above, the covariance of the noise afterpost-processing is equal to Σ⁻², which implies that this received noisehas been diagonalized. This in turn means that the subspace that isorthogonal to the range space of the interference noise vector, namelythe nullspace of the interference noise, now contains some of thechannels of the multiline system. Moreover, due to the addition of thesplit-pair receivers, the dimension of the subspace that is free ofcrosstalk noise has been increased.

Hence, compared to a multiline system without split-pair receivers, morechannels are now free of noise and can carry significantly higherbitrates than the channels that are in the range space of theinterference noise, which continue to be impaired. In other words, thesplit-pair MIMO processing of FIG. 5 increases the total number ofavailable channels and then restricts the effect of the interferencenoise to the minimum possible number of channels, and therefore isoptimal with respect to the criterion of maximizing the overall capacityof the multiline transmission system.

FIG. 6 illustrates exemplary MIMO post-processing process 600 once aprocessing architecture is computed according to one embodiment of thepresent invention. Process 600 commences at block 601. At processingblock 610, each transceiver in the split-pair multiline transmissionsystem receives differential signal vectors including crosstalk. Thecrosstalk received is not limited in origin and may be alien crosstalkor crosstalk from sources outside or external to the split-pairmultiline transmission system. The split-pair receiver receives a signalvector including crosstalk from each transceiver at processing block620. The MIMO post-processing unit receives signals includinginterference signals and vector signals from each transceiver and eachsplit-pair receiver at processing block 630. At processing block 640, aMIMO post-processor with a receiver converts each of the received signalvectors into received symbol vectors. The received symbols are decodedat processing block 650. The process completes at block 699.

FIG. 7 illustrates an exemplary block diagram of a computer system 700representing an integrated multi-processor, in which elements of thepresent invention may be implemented. Computer system 700 can be used toperform the method 600 as referenced in FIG. 6. One embodiment ofcomputer system 700 comprises a system bus 716 for communicatinginformation, and a processor 718 coupled to bus 716 for processinginformation. Computer system 700 further comprises a random accessmemory (RAM) or other dynamic storage device 710 (referred to herein asmain memory), coupled to bus 716 for storing information andinstructions to be executed by processor 718. Main memory 710 also maybe used for storing temporary variables or other intermediateinformation during execution of instructions by processor 718. Computersystem 700 also may include a read only memory (ROM) and/or other staticstorage device 712 coupled to bus 716 for storing static information andinstructions used by processor 718.

A data storage device 714 such as a magnetic disk or optical disc andits corresponding drive may also be coupled to computer system 700 forstoring information and instructions. Computer system 700 can also becoupled to a second I/O bus 722 via an I/O interface 720. A plurality ofI/O devices may be coupled to I/O bus 722, including a display device708, an input device (e.g., an alphanumeric input device 706 and/or acursor control device 704). For example, video news clips and relatedinformation may be presented to the user on the display device 708.

The communication device 702 is for accessing other computers (serversor clients) via a network. The communication device 702 may comprise amodem, a network interface card, or other well-known interface device,such as those used for coupling to Ethernet, token ring, or other typesof networks.

In the foregoing specification, the invention has been described withreference to specific embodiments. It will, however, be evident thatvarious modifications and changes can be made without departing from thebroader spirit and scope of the invention as set forth in the claims.The specification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense.

1. A method reducing signal distortion in a multiple line transmissionsystem, the method comprising: using one or more split-pair receivers ina multiline communications system to identify crosstalk on a pair oftransceivers coupled to the split pair receivers, wherein each splitpair receiver receives a signal including the crosstalk from eachtransceiver and provides a corresponding signal vector to a postprocessing unit; and performing MIMO post-processing on signal vectorsreceived at a receiver from each transceiver and each split-pairreceiver while minimizing crosstalk on pairs of lines in the multilinecommunications system with a frequency equalizer.